C＃Windows Universal应用未检测到异步方法

Question

So I'm trying to write a simple universal app to get the price of bitcoin from the web. I have an async method that I got from here to get the json from a url and put it into a string. Here is where I called the method:

        public App()
        {
            this.InitializeComponent();
            this.Suspending += OnSuspending;

            CoinPriceBackend CP = new CoinPriceBackend();

            string response = await GetFromAPI();
        }

And this is the method:

        async Task<string> GetFromAPI()
        {
            try
            {
                //Create HttpClient
                HttpClient httpClient = new HttpClient();

                //Define Http Headers
                httpClient.DefaultRequestHeaders.Accept.TryParseAdd("application/json");

                //Call
                string ResponseString = await httpClient.GetStringAsync(
                    new Uri("https://api.bitcoinaverage.com/ticker/GBP/"));
                //Replace current URL with your URL

                return ResponseString;
            }

            catch (Exception ex)
            {
                return "ERROR: " + ex;
            }
        }

I get the error

'The 'await' operator can only be used within an async method. 
Consider marking this method with the 'async' modifier and changing its return type to 'Task'.'

But the method is async... How can I fix this?

Thanks!

Answer 1

But the method is async

Take a closer look at the error message; it's not talking about GetFromAPI - it's talking about App .

However, as others have pointed out, constructors cannot be marked async .

I'm trying to write a simple universal app

Universal Windows apps - like all other modern platforms - cannot block the UI thread for I/O-based operations. The user experience is just too bad, and there are tests in most app stores to auto-reject apps that do this.

Put another way: App is called (presumably) on application startup. When the user launches your app, it has to start up quickly and show something ASAP . Waiting for a download to complete is simply not an option.

So, to really fix this, you need to just start the download (not waiting for it to complete) and initialize your application to a "loading" state - showing a spinner or "Loading..." message or whatever. Then, when the download completes, update your app to display what you need to.

I have a blog post on async constructors and an article series on async MVVM (if you're doing MVVM), but a really basic approach could look something like this:

public Task Initialization { get; }
public string Value { get; private set { /* code to raise PropertyChanged */ } }

public App()
{
  this.InitializeComponent();
  this.Suspending += OnSuspending;

  CoinPriceBackend CP = new CoinPriceBackend();

  Value = "Loading..."; // Initialize to loading state.
  Initialization = InitializeAsync();
}

private async Task InitializeAsync()
{
  try
  {
    string response = await GetFromAPI();
    ...
    Value = response; // Update data-bound value.
  }
  catch (Exception ex)
  {
    ... // Display to user or something...
  }
}

Answer 2

When you use an await within a function you should define the function as async .

But for App() constructor you will be unable to do that. You can defined another function from which you can call your function which return string .

Like this

public App()
{
    CallApi();
}

private async void CallApi()
{
    response = await GetFromAPI();
}

Answer 3

C# does not allow constructors to be marked as async.

You have three main options:

1) refactor to call this method in async event handler;

2) spawn a new thread using Task.Run and run async code there. This could lead to issues of marshalling result back to UI thread and assigning value to some UI element;

3) make it synchronous (blocking) call. This probably is the easiest option.

You will have to make following changes.

   string response = GetFromAPI().Result;

Note that this will probably cause deadlock as Task will try to reaume in the main thread, which is already locked by call to '.Result', therefore you need another change. Also, there is no point

Task<string> GetFromAPI()
{
    ....

    return httpClient.GetStringAsync(new Uri("https://api.bitcoinaverage.com/ticker/GBP/")).ConfigureAwait(false);

    ...
}